Fibers having a diameter of no greater than about 100 microns (.mu.m), and particularly microfibers having a diameter of no greater than about 50 .mu.m, have been developed for a variety of uses and with a variety of properties. They are typically used in the form of nonwoven webs that can be used in the manufacture of face masks and respirators, air filters, vacuum bags, oil and chemical spill sorbents, thermal insulation, first aid dressings, medical wraps, surgical drapes, disposable diapers, wipe materials, and the like. The fibers can be made by a variety of melt processes, including a spunbond process and a melt-blown process.
In a spunbond process, fibers are extruded from a polymer melt stream through multiple banks of spinnerets onto a rapidly moving, porous belt, for example, forming an unbonded web. This unbonded web is then passed through a bonder, typically a thermal bonder, which bonds some of the fibers to neighboring fibers, thereby providing integrity to the web. In a melt-blown process, fibers are extruded from a polymer melt stream through fine orifices using high air velocity attenuation onto a rotating drum, for example, forming an autogenously bonded web. In contrast to a spunbond process, no further processing is necessary.
Fibers formed from either melt process can contain one or more polymers, and can be of one or more layers, which allows for tailoring the properties of the fibers and products produced therefrom. For example, melt-blown multilayer microfibers can be produced by first feeding one or more polymer melt streams to a feedblock, optionally separating at least one of the polymer melt streams into at least two distinct streams, and recombining the melt streams, into a single polymer melt stream of longitudinally distinct layers, which can be of at least two different polymeric materials arranged in an alternating manner. The combined melt stream is then extruded through fine orifices and formed into a highly conformable web of melt-blown microfibers.
Thermoplastic materials, such as thermoplastic elastomers, can be used in the melt processing of fibers, particularly microfibers. Examples of such thermoplastic materials include polyurethanes, polyetheresters, polyamides, polyarenepolydiene block copolymers such as those sold under the trade designation KRATON, and blends thereof. It is known that such thermoplastic materials can be either adhesive in nature or can be mixed with tackifying resins to increase the adhesiveness of the materials. For example, webs of microfibers made using a melt-blown process from pressure-sensitive adhesives comprising block copolymers, such as styrene-isoprene-styrene block copolymers available under the trade designation KRATON, are disclosed in International Publication No. WO 96/16625 (The Proctor & Gamble Company) and U.S. Pat. No. 5,462,538 (Korpman). Also, webs of multilayer microfibers made using a melt-blown process from tackified elastomeric materials, such as KRATON block copolymers, are disclosed in U.S. Pat. Nos. 5,176,952 (Joseph et al.), 5,238,733 (Joseph et al.), and 5,258,220 (Joseph).
Thus, nonwoven webs are known that are formed from melt-processed fibers having a variety of properties, including adhesive and nonadhesive properties. Not all polymeric materials, however, are suitable for use in melt processes used to make such fibers. This is particularly true for materials that are pressure-sensitive adhesives, typically because the extreme conditions used in melt processes can cause significant breakdown of molecular weights of the polymers resulting in low cohesive strength of the fiber. Thus, there is still a need for nonwoven webs of fibers having a variety of properties, particularly pressure-sensitive adhesive properties.